A goniospectrometer (also spectrogoniometer, gonioreflectometer, reflection goniometer, reflectance goniometer or else concisely just goniometer, wherein a goniometer is fundamentally a device for angle determination) is a device for measuring the reflection behaviour of a natural surface, for example a vegetation-covered subsurface. Generally, the bidirectional reflectance distribution function (BRDF) is determined for a given light-incidence and observation direction, i.e. the reflection factor is determined as a function of the position of the sun and the position of the optical unit. In this case, the azimuth angle (angular direction of the sun, measured from a cardinal direction (North inter alia) at)(0° in the clockwise direction to 360°) and the zenith angle (angular position of the sun above the horizon, measured from the horizon)(90° to 0° above the object) are considered as parameters in the observation geometry. The BRDF is a fundamental optical property of the reflecting material. Due to the large variability of the BRDF, depending on the material properties of the surface, practical applications cannot be based on individual nadir measurements of this surface. BRDF models are required which describe the characteristic properties of the object group (grassland, agricultural surface at various phenological stages, etc.). Satellite-based earth observation (since the end of 1970) has required increased investigation of the directed reflection properties of materials and thus the development of models for describing the directed reflection for BRDF correction of spectral satellite data. For example, the vegetation in a permafrost region, e.g. the Siberian Tundra, shows a strongly anisotropic behaviour, i.e. a direction-dependent reflection behaviour of sunlight. The strength of the anisotropy in this case depends on ground moisture, solar zenith angle and zenith angle of the optical unit of the goniospectrometer. Anisotropic reflection behaviour (also anisotropic reflectance or differential spectroscopic reflectance) therefore strongly influences the BRDF.
The BRDF correction of data from wide-angle satellites and narrow-angle satellite missions using oblique imaging methodology. Due to the technical development of satellite mission platforms with greater manoeuvrability and technical improvements, current and future planned national and international satellite missions increasingly use oblique imaging technology, in order to make data acquisition possible in spite of cloud cover. Due to novel technology and mobile platforms, the percentage of oblique images from narrow band satellites is growing disproportionately compared to nadir images. BRDF correction is necessary when quantitative and qualitative parameters of the surface are derived. Wide angle satellites are satellite missions in a high orbit using a broad imaging strip which requires BRDF correction outside of the nadir strip. Wide angle satellite missions are principally long-term satellite missions (NOAH, AVHRR satellite missions since the 70s, NASA MODIS (since 2000)), which provide global parameter maps, e.g. vegetation index, leaf area index, vegetation classes.
The background for field measurements is the fact that climate-induced changes for example in a permafrost region are shown in the change of the surface temperature regime and the moisture regime. Vegetation coverage and vegetation development are therefore influenced in a secondary manner. Permafrost regions make up almost a quarter of the land surface north of the equator and are therefore of global significance, but difficult to access and hitherto only slightly explored scientifically. Hyperspectral remote sensing using satellite missions offers great potential here, in order to deliver models for carbon balancing and for calculating energy and greenhouse gas flows and for exploration of raw materials, the occurrence of which influence the natural surface in a characteristic manner, and for exploring suitable regions for cultivation, for example for grain or oleiferous plants. Novel satellites are also able to produce oblique images of the earth's surface by pivoting, which leads to a multiplying of measurement points compared to simple vertical images. The influence of anisotropy in these oblique images has hitherto not or not satisfactorily been taken into account. To determine whether a correction of oblique images is necessary, in-situ measurements must be carried out under real imaging conditions in the terrain. Goniospectrometers have been used for this for years. A transportable goniospectrometer (also field or site goniospectrometer) is particularly suitable for field use (a laboratory use is however also readily possible). In this case, it has however been shown that known site goniometers are only of limited suitability for custom requirements, particularly logistics, such as transport in inaccessible terrain without roads, small team sizes and high air humidity and cold (which place high demands on operability), in permafrost and Arctic regions. A fundamental distinction can be made between goniospectrometers with constant observation center (measurement location) and goniospectrometers with constant optical unit position. Goniospectrometers with constant observation center for the most part consist of an azimuth ring (corresponds to cardinal direction horizon), on which a zenith ring (corresponds to daily course of the sun) is fastened, which can be moved whilst guided through the azimuth ring. A displaceable slide is fastened on the zenith ring, which carries the optical unit for radiometric measurement and can fix the same freely at a zenith angle. Goniospectrometers with a constant optical unit position for the most part consist of an arm, on which an optical unit is fastened, which can be adjusted to various angles with respect to the object.
“Polarised Multiangular Reflectance Measurements Using the Finnish Geodetic Institute Field Goniospectrometer” by J. Suomalainen et al. (in Sensors 2009, 9, 3891-3907) describes a transportable goniometer (acronym FIGIFIGO) that is used for the radiometric measurement of the reflection behaviour of natural subsurfaces, measurements in the snow are shown. The FIGIFIGO belongs to the category of goniospectrometers with constant observation center and consists of a central main pillar, which has a support in the form of a box opposite the natural surface to be measured, here a blanket of snow as a natural subsurface. The main pillar is laterally pivotably arranged on a longitudinal side of the box. The main pillar is connected at the upper pillar end thereof via a screw connection to the fixed cantilever end of a cantilever. Furthermore, the known goniometer has a spectrometer with an optical unit and a sensor, wherein the optical unit is connected to the sensor via an optical fiber. The optical unit is arranged at the location of the screw connection, a rotatable mirror is located at the end of the cantilever, by means of which the reflections of the blanket of snow are diverted into the optical unit. Thus, the constant observation center is only reached by the mirror and not by the optical unit. The screw connection consists of a type of open shells, which surrounds the round housing and clamped by a screwed connection. The sensor is accommodated together with an analysis unit in the box on the lower pillar end of the main pillar. In a position of the box on the blanket of snow, a measurement series can be carried out using the device in a plane parallel to the front edge of the at various viewing angles (tilting the main pillar along the front edge of the box) in a measurement location (field of vision). In order to be able to carry out measurements in the measurement location, the entire box must be rotated.
The classic field goniospectrometer with constant observation center with the above-described construction made up of azimuth and zenith ring is described in the publication: “The improved Dual-view Field-Goniometer System FIGOS” by J. Schopfer et al. (in Sensors 2008, 8, pp. 5120-5140) and is mentioned here for the sake of the completeness of the overview. The constructively stable, but also space-consuming construction can clearly be seen. Many further site and laboratory goniospectrometers based on the model of the FIGOS have been developed.
The basic type of the goniospectrometer with constant observation center is described in the publication: “A low-cost field and laboratory goniometer system for estimating hyperspectral bidirectional reflectance” by C. A. Coburn et al. (in Can, J. Remote Sensing, Vol. 32, No. 3, pp. 244-253, 2006). It consists of a closed azimuth ring, on which a half zenith ring is rotatably arranged. A slide with the optical unit, which can be travelled to any point on the half-sphere shell, runs on the half zenith ring. A completely automated goniospectrometer with constant observation center is described in the publication: “Automated spectrogoniometer: A spherical robot for the field measurement of the directional reflectance of snow” by T. Painter et al. (in Rev. Sci. Instrum., Vol. 74, No. 12. December 2003, pp. 6179-5177). Here, only a quarter zenith arc, which carries two further arc sections which are each rotatably mounted at the end thereof, is provided over an azimuth arc. The optical unit is arranged at the end of the second arc section. The HRDF (hemispherical directional reflectance function), which in contrast with BRDF also takes account of diffuse reflection of the natural surface, is measured. A good overview of the various developments in the field of field spectrometry up to 2007 is given in the publication: “Progress in field spectroscopy” by E. J. Milton et al. (in Remote Sensing of Environment (2007), doi:10.1016/ j.rse.2007.08001). The various efforts of the user to make the goniospectrometer as light and easily transportable as possible can easily be seen.
A portable goniometer for characterising artificial surfaces is known from WO 2006/056647 A1, the main pillar of which is mounted with three legs as supports. The main pillar can be moved vertically and horizontally along these supports. At the upper pillar end, the main pillar is connected via a screw connection to the fixed cantilever end of a relatively short cantilever. The free cantilever end thereof is securely connected to the center of an arc. The arc carries an X-ray source and an optical unit in a fixed position. By rotating the cantilever about the longitudinal axis thereof, the angles of incidence and observation can be changed relatively to the measurement location in an angular range and measured in the laboratory.
An apparatus for simulating insolation in the laboratory is known from DE 26 43 647 A1, in which an arc pivotable about the horizontal axis of the irradiated object is provided, which carries a further arc with orthogonal alignment to the first arc. The further arc can be displaced along the first arc. A displaceable slide with a light source is arranged on the further arc. The irradiated object is arranged on a rotary table, so that all directed insolations onto every location of the irradiated object can be simulated by means of the interaction of individual rotations, pivoting movements and displacements.
A transportable diffractometer for laboratory measurement with a main pillar is known from EP 1 470 413 B1, which is arranged on a mobile framework. A vertical cantilever is fastened on the main pillar, which is securely connected via a rotatable suspension to an arc. In addition to an x-ray source, a detector, which can be displaced on the arc, is also fastened on the arc. Furthermore, the arc can also be tilted through the plane along the vertical cantilever, so that any desired angular adjustments can be taken up on a full circle about the measurement object.